LOS ANGELES — One of Michael Jackson’s bodyguards had barely stepped into the singer’s bedroom when he heard a scream. “Daddy!” Jackson’s young daughter cried.

A few feet away, the singer lay motionless in his bed, eyes slightly open. His personal doctor, Conrad Murray, was trying to revive him when he saw that Jackson’s eldest children were watching.

“Don’t let them see their dad like this,” Murray said, the first of many orders that bodyguard Alberto Alvarez testified Thursday that he heeded in the moments before paramedics arrived at Jackson’s home in June 2009.

What happened next — after Alvarez said he ushered Jackson’s eldest son and daughter from the room — is one of the key pieces of prosecutors’ involuntary manslaughter case against Murray.

According to Alvarez, Murray scooped up vials of medicine from Jackson’s nightstand and told the bodyguard to put them away. “He said, ‘Here, put these in a bag,”’ Alvarez said.

Alvarez complied. He also placed an IV bag into another bag.

On the third day of the trial, prosecutors tried to show that Murray, who has pleaded not guilty, delayed calling authorities and that he was intent on concealing signs that he had been giving the singer doses of the anesthetic propofol.

Alvarez said he thought Murray might be preparing to take the items to the hospital, but didn’t question him.

The bags never made it to the hospital, and prosecutors claim Murray repeatedly lied to emergency personnel and did not tell them he had been giving Jackson doses of the drug as a sleep aid.

If convicted, Murray, 58, could face up to four years in prison and lose his medical license.

Defense attorney Ed Chernoff questioned whether there was enough time for Alvarez to shield Jackson’s children, survey the room and stow away the drugs in the brief period that phone records show he was in the home before calling emergency responders.

The bodyguard insisted there was, telling the attorney, “I’m very efficient, sir.”

Chernoff was not convinced, questioning whether 30 seconds was enough time for the dramatic sequence to play out. Alvarez assured him there was.

The defense attorney also challenged Alvarez’s recollection, asking whether the collection of the vials happened after paramedics had come and whisked Jackson to a nearby hospital. Alvarez denied it happened after he called 911.

Chernoff questioned why Alvarez didn’t tell authorities about Murray’s commands to bag up the medication immediately after Jackson died, but instead waited until two months after the singer’s death. The bodyguard said he didn’t realize its significance until seeing a news report in late June in which he recognized one of the bags detectives were carrying out of Jackson’s mansion.

The burly Alvarez became emotional as the 911 call was played for jurors. Jackson’s mother, Katherine, appeared distraught and her son, Randy, huddled next to her and put his arm around her.

“Was that difficult to hear?” prosecutor David Walgren asked.

“It is,” Alvarez replied.

After hanging up with dispatchers, Alvarez said he performed chest compressions on Jackson while Murray gave the singer mouth-to-mouth resuscitation. The doctor remarked it was his first time performing the procedure.

Alvarez recalled seeing Murray at the hospital where Jackson was taken and sitting next to the emergency room.

“‘I wanted him to make it,”’ Alvarez quoted Murray as saying. “‘I wanted him to make it.”’

Alvarez’s testimony allowed Walgren to present jurors directly with a bottle of propofol that they’ve heard much about throughout the previous two days of the trial.

Jurors intently looked at the bottle, which appeared to still contain some liquid.

When he entered the bedroom, Alvarez said, he saw Jackson’s eyes were open and was surprised to see the singer was wearing a condom catheter, a medical device that allows one to urinate without having to get up.

Alvarez testified that Murray only told him Jackson had a “bad reaction.” Jackson’s personal assistant, who testified Wednesday, said Murray told him the same thing.

Alvarez said it was a stunning scene, a far cry from the night before when the bodyguard stood backstage at Staples Center, sneaking peeks of Jackson performing during what would be his final rehearsal.

“He was very happy,” Alvarez testified. “I do recall he was in very good spirits.”

In another effort to cast doubt on the bodyguard’s testimony, Murray asked whether Alvarez, another bodyguard, Faheem Muhammad, and Jackson’s assistant, Michael Amir Williams, colluded before being interviewed by detectives two months after Jackson’s death.

The three men, who were the first to interact with Murray after Jackson stopped breathing, have denied the accusation.

Others who will testify about events at Jackson’s rented mansion are Kai Chase, a chef who spoke to Murray briefly on the morning of Jackson’s death, and paramedics who also tried to revive the singer.

The medics believed Jackson was already dead by the time they arrived, but Murray insisted the performer be taken to a hospital for additional resuscitation efforts.

Walgren asked whether anything good had happened to Alvarez as a result of his experience in Jackson’s bedroom.

“No sir,” Alvarez responded.

Media outlets offered him up to $500,000 for interviews, but Alvarez said he always refused. “It’s caused a lot of financial problems,” he said, starting to choke up. “I went from a great salary to hardly anything.”